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Essentials of Geology, 11e

Mass Wasting: The Work of Gravity

Chapter 8

Stanley Hatfield

Southwestern Illinois College

Jennifer Cole

Northeastern University

Instructor – Jennifer Barson

Spokane Falls Community College

Geology 101

Figure 8.15

Mass Wasting and Landform Development

• Mass wasting refers to the downslope movement of rock, regolith, and soil under the direct influence of gravity

– Does not require a transporting medium

• Role of mass wasting

• Geologic process that often follows weathering

• Combined effects if mass wasting and running water produce stream valleys

Mass Wasting and Landform Development

• For mass wasting to occur, there must be a slope angle

• Most rapid events occur in areas of rugged, geologically young mountains

• As a landscape ages, less dramatic downslope movements occur

Controls and Triggers of Mass Wasting

• Gravity is the controlling force

• A ‘trigger’ is the event that initiates movement

– Something has caused the slope to cross the threshold from stable to unstable

• 4 Important triggers include

– Saturation of the material with water

• Diminishes particle cohesion

• Water adds weight

Oversaturation

Figure 8.4

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Hurricane Mitch – Honduras, 1998

Figure 8.3

Controls and Triggers of Mass Wasting

• 4 Important triggers include

–Oversteepening of slopes

• Stable slope angle (angle of repose) is different for various materials

• Oversteepened slopes are unstable

–Removal of anchoring vegetation

• Root systems bind soil and regolith

• Wildfires are serious risks

Controls and Triggers of Mass Wasting

• 4 Important triggers include

– Ground vibrations from earthquakes

• May cause extensive property damage

• Can cause liquefaction – water saturated surface materials behave as fluid-like masses that flow

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Figure 8.8

Classification of Mass Wasting Events

• Landslides without triggers

• Slope materials weaken over time

• Random events that are unpredictable

• Look at 4 processes • Slump, rockslide, debris flow, and earthflow

• Generally each event is classified by

• Type of material involved

• Kind of motion displayed

• Velocity of the movement

Classification of Mass Wasting Events

• Generally each event is classified by

– Type of material involved

• Debris – large amount of water

• Mud – more water, finer grained

• Earth – regolith/soil

• Rock

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Classification of Mass Wasting Events

• Generally each event is classified by • Type of motion

– Fall (free-falling pieces)

– Slide (material moves along a surface as a coherent mass)

– Flow (material moves as a chaotic mixture)

• The velocity of the movement – Fast - rockslide

– Slow - creep

Figure 8.14

Forms of Mass Wasting

• Slump

–Movement of a mass of rock or uncon-solidated material as a unit along a curved surface • Crescent-shaped scarp (head)

• Occurs along oversteepened slopes – Anchor material at the base is removed

• Occurs along overloaded slopes – EX - Clay-rich material underlies sandstone

A Slump at Point Fermin, CA

Figure 8.15

Forms of Mass Wasting

• Rockslide

–Blocks of bedrock slide down a slope

–Generally very fast and destructive

Figure 8.16

Rockslide

Figure 8.16

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Forms of Mass Wasting

• Debris flow (mudflow) – Consists of soil and regolith with water

– Often confined to channels

– Serious hazard in dry areas with heavy rains

– Debris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars

• Mudflow – Also soil and regolith with larger amount of water

– Also confined to channels

Debris Flow (Mudslide)

Figure 8.18

Photo by D. R. Crandell, U.S. Geological Survey

Forms of Mass Wasting Figure 8.19

Forms of Mass Wasting

• Earthflow

– Form on hillsides in humid regions

– Water saturates the soil

– Commonly involve materials rich in clay and silt

Figure 8.20a

An Earthflow on a Newly Formed Slope

Figure 8.20

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Forms of Mass Wasting

• Slow movements

–Creep

• Gradual movement of soil and regolith downhill

• Aided by the alternate expansion and contraction of the surface material

Figure 8.21

Creep

Creep Forms of Mass Wasting

• Slow movements

– Solifluction

• Promoted by a dense clay hardpan or impermeable bedrock layer

• Common in regions underlain by permafrost

• Can occur on gentle slopes

Figure 8.22

Ground Subsidence in Alaska Due to Permafrost

Figure 8.23 A/B